CCITT Instructions for the international telephone service (1 October 1985), ITU,Geneva,1985.
适用范围
In 1934 (CCIF White Book, Volume III,Xth Plenary Assembly,Budapest,1934),a signalling current having a frequency of 500 Hz ± 2%,interrupted at a frequency of 20 Hz ± 2% was provisionally chosen for manually-operated international circuits.